Design of a radix-8/4/2 FFT processor for OFDM systems

Please through yourself with this paper which provides deeper insight into the following:

The FFT processor is based on radix-8 FFT algorithm and also supports radix-4 or radix-2 FFT computation. The pipelined radix-8/4/2 butter y unit computes radix-8 FFT algorithm basically. For the limitation of FFT length, if it cannot run radix-8 FFT algorithm at the last stage then it computes radix-4 or radix-2 FFT algorithm.

PAPER : Design of a radix-8/4/2 FFT processor for OFDM systems